The Sickle Cell Center at the Children's Hospital of Philadelphia (SCC-CHOP) comprises the state-funded Clinical Sickle Cell Disease Program and the NIH-supported Comprehensive Sickle Cell Center. This proposal for the establishment of a Clinical Center within the Sickle Cell Disease Clinical Research Network (SCO-CRN) combines the strengths of both components of the SCC-CHOP - the experienced clinical management and care coordination of SCO in children up to 21 years of age and long record of clinical research in SCO and other hemoglobinopathies. The SCC-CHOP has a long history of participation in multi- center collaborative clinical studies as well as local single institution studies. The proposed Clinical Center will have access to the extensive resources at CHOP for both clinical care coordination and clinical research. Our Clinical Center will recruit from the 705-patient Sickle Cell Disease Clinical Program at CHOP and adults from the 110-patientSickle Cell Program at Mercy Hospital of Philadelphia. Transfusion therapy is the most commonly used intervention in people with SCD who have developed major clinical complications. However, it is in only the management of stroke that this therapy has been tested in well-organized controlled studies. In addition, the increasing use of transfusion therapy in SCD predicts future problems with iron overload. We propose two Clinical Trials both of which take advantage of our extensive experience in transfusion therapy and transfusional iron overload in hemoglobinopathies. Study 1 seeks to compare the effect of partial exchange transfusion (PET) with that of simple transfusion plus chelation therapy on the degree of iron overload in patients with SCD. In addition, the study seeks to determine the relationship between total body iron burden and liver and cardiac iron loading in chronically transfused patients with SCD, and the relationship between organ-specific iron overload and organ dysfunction in SCD. Study 2 will compare the effect of hydroxyurea (HU) therapy to that of chronic red cell transfusion (CRCT) therapy in reducing the frequency of recurrent acute chest syndrome (ACS) in patients with sickle cell disease SS (SCD-SS) without asthma and a similar study in those with SCD-SS and asthma. In an accompanying application, This center is also applying for a Patient Outcomes Core for the SCO-CRN. Sickle cell disease is one of the most common inherited diseases in America. These studies have the potential to improve the health and prolong the life expectancy of patients with the most severe complications of the disease.